What are the biggest potential drawbacks of the IPF5100/iPF5000?

There are only a few, but they could be deal breakers for you depending on your needs:

• Very large size compared to Epson 3800 (bigger than even the Epson 4800)
• Full borderless printing on sheets is not supported
• Media Types locked out for some paper paths
• Smallest paper size 8X10
• Somewhat greater gloss differential than Ultrachrome K3 (Epson 3800/4800) Note: This may not apply to iPF5100. We need more information.
• Inks less water resistant than Ultrachrome K3 on some papers
• Steeper learning curve for those already accustomed to Epson printers